In internal combustion engines, camshafts are used for actuating the gas exchange valves. Camshafts are mounted in the internal combustion engine in such a way that cams attached to them bear against cam followers, for example bucket tappets, drag levers or rocker arms. When the camshaft is set in rotation, the cams roll on the cam followers which in turn actuate the gas exchange valves. Thus, owing to the position and shape of the cams, both the opening duration and amplitude, but also the opening and closing time point of the gas exchange valves are defined.
Modern engine concepts tend toward a variable design of the valve drive. On the one hand, the valve stroke and valve opening duration are to be capable of being configured variably up to the complete cut-off of individual cylinders. For this purpose, concepts, such as switchable cam followers, variable valve drives or electro-hydraulic or electric valve actuations, are provided. Furthermore, it has proved advantageous to be able to exert influence on the opening and closing times for the gas exchange valves while the internal combustion engine is in operation. It is likewise desirable to be able to influence the opening and closing time points of the inlet and outlet valves separately so that, for example, a defined valve overlap can be set in a directed way. By the opening and closing time points of the gas exchange valves being set as a function of the current characteristic map range of the engine, for example of the current rotational speed or the current load, the specific fuel consumption can be lowered, the exhaust gas behavior can be influenced positively and the engine efficiency, maximum torque and maximum power can be increased.
The variability in the gas exchange valve time control, as described, is brought about by means of a relative change in the phase position of the camshaft with respect to the crankshaft. In this case, the camshaft is mostly drive-connected to the crankshaft via a chain, belt or gearwheel mechanism or identically acting drive concepts. Between the chain, belt or gearwheel mechanism driven by the crankshaft and the camshaft is mounted a camshaft adjuster which transmits the torque from the crankshaft to the camshaft. In this case, this device for varying the control times of the internal combustion engine is designed in such a way that, while the internal combustion engine is in operation, the phase position between the crankshaft and camshaft can be maintained reliably, and, if desired, the camshaft can be rotated within a certain angular range with respect to the crankshaft.
In internal combustion engines with a camshaft in each case for the intake and the exhaust valves, these may be equipped in each case with a camshaft adjuster. As a result, the opening and closing times of the intake and exhaust gas exchange valves can be displaced relative to one another in time and the valve time overlaps can be set in a directed way.
The seat of modern camshaft adjuster is generally located at the drive-side end of the camshaft. It consists of a crankshaft-fixed driving wheel, of a camshaft-fixed driven element and of an adjusting mechanism transmitting the torque from the driving wheel to the driven part. The driving wheel may be designed as a chain wheel, belt wheel or gearwheel and is connected fixedly in terms of rotation to the crankshaft by means of a chain, a belt or a gearwheel mechanism. The adjusting mechanism may be operated electromagnetically, hydraulically or pneumatically. It is likewise conceivable to attach the camshaft adjuster to an intermediate shaft or to mount it on a nonrotating component. In this case, the torque is transmitted to the camshafts via further drives.
Electrically operated camshaft adjusters consist of a driving wheel which is drive-connected to the crankshaft of the internal combustion engine, of a driven part which is drive-connected to a camshaft of the internal combustion engine and of an adjusting mechanism. The adjusting mechanism is a three-shaft mechanism with three components rotatable with respect to one another. In this case, the first component of the mechanism is connected fixedly in terms of rotation to the driving wheel and the second component is connected fixedly in terms of rotation to the driven part. The third component is designed, for example, as a toothed component, the rotational speed of which can be regulated via a shaft, for example by means of an electric motor or a braking device.
The torque is transmitted from the crankshaft to the first component and from there to the second component and consequently to the camshaft. This takes place either directly or with the third component being interposed.
By the rotational speed of the third component being suitably regulated, the first component can be rotated with respect to the second component, and consequently the phase position between the camshaft and crankshaft can be varied. Examples of three-shaft mechanisms of this type are inner eccentric mechanisms, double inner eccentric mechanisms, harmonic drives, swashplate mechanisms or the like.
To control the camshaft adjuster, sensors detect the characteristic data of the internal combustion engine, such as, for example, the load state, the rotational speed and the angular positions of the camshaft and crankshaft. These data are fed to an electronic control unit which, after comparing the data with a characteristic map of the internal combustion engine, controls the adjusting motor of the camshaft adjuster.
DE 102 36 507 discloses a device for varying the control times of an internal combustion engine, in which torque transmission from the crankshaft to the camshaft and the adjusting operation are implemented by means of a swashplate mechanism. The device consists essentially of a camshaft wheel, of a camshaft-fixed rotary disk and of a swashplate mechanism. The camshaft wheel is drive-connected to a crankshaft and is produced in one piece with a housing. The swashplate is mounted on an adjusting shaft at a defined angle of incidence and is drive-connected to the housing.
The swashplate and the rotary disk are provided, on their axial side faces in each case facing the other component, in each case with a bevel wheel toothing in the form of a toothed rim. In this case, the swashplate and the rotary disk are arranged in such a way that, because of the mounting of the swashplate on the adjusting shaft at a specific angle of incidence, an angular segment of the toothing of the swashplate engages into an angular segment of the toothing of the rotary disk. There is, in this case, a difference in the number of teeth of the bevel wheel toothings.
The adjusting shaft is drive-connected to a drive unit, for example an electric motor, which drives it at a continuously regulatable rotational speed. A rotation of the adjusting shaft in relation to the rotary disk leads to a wobbling rotation of the swashplate and consequently to a rotation of the engaged angular segment in relation to the rotary disk and to the swashplate. On account of the different number of teeth of the bevel wheel toothings, this leads to a relative rotation of the camshaft with respect to the crankshaft.
If the amount of the relative angle of rotation between the camshaft and crankshaft overshoots a specific value, the internal combustion engine can no longer operate reliably. In the event of an extreme offset, the pistons may knock against the open gas exchange valves, thus leading to engine damage. Phase positions of this kind may occur, for example, due to a failure of the control unit or of the drive of the adjusting shaft or in the case of damage to the device itself. In order to prevent the offset between the angle of rotation of the crankshaft and the angle of rotation of the camshaft from becoming too great, means are provided for limiting the relative angle of rotation of the camshaft with respect to the crankshaft. For this purpose, the rotary disk connected at the camshaft is provided with a clearance, into which a stop arranged on the camshaft wheel engages. The stop may be produced in one piece with the camshaft wheel or be fastened to the latter.
This embodiment has the disadvantage that the stop has to be designed with a very high mass in order to withstand the high forces occurring in the event of a malfunction of the device. This leads to a large mass of the device and to high production costs. Furthermore, this principle proves to be inflexible in terms of the manufacture of various devices having different adjustment angle ranges.